Fluid bilevel sensors have been long used in such applications as control devices for sump pumps, mine drainage pumps, bilge pumps, and toilet tanks. They sense when a first level of fluid is attained, then provide a control signal which frequently is used to control a valve or pump which tends to change the fluid level in the desired direction. When they sense that a second level of fluid is attained, they provide a second control signal which usually alters the condition of valve or pump. For example, in the case of the pump control devices mentioned above, when a first fluid level is attained, the bilevel sensors provide an electrical output signal which energizes the pump to begin pumping fluid into or out of the measured fluid. When the fluid level is changed to the second level, the fluid bilevel sensor removes the electrical output signal and deenergizes the pump.
An extensive art has developed in single-level and bilevel fluid sensors. For example, Booth et al in U.S. Pat. No. 2,927,175, Lewis et al in U.S. Pat. No. 3,560,918, Hocking et al in U.S. Pat. No. 3,691,522, and Putt in U.S. Pat. No. 3,678,490 all disclose fluid level sensors for a variety of purposes.
The prior art fails to teach a fluid level sensor, either single level or bilevel, which uses a differential float. The prior art also fails to teach the use of a thermal time delay with a fluid level sensor.